1. Field
This invention relates to ceramic materials which are useful in many applications, but especially to those which are particularly useful in electrolytic systems for simultaneous production of caustic and acids by electrolysis of an aqueous alkali metal (e.g., sodium) salt solution.
2. State of the Art
Electrolytic systems for use in producing caustic from salt solutions have been disclosed. In such systems, various materials have been suggested for use as an electrolytic membrane positioned between an anolyte chamber and a catholyte chamber for transportation of ions therethrough. Such materials include ceramic materials alone, polymeric materials, and combinations of ceramic and polymeric materials.
The known advantage of polymeric materials as electrolytes in the electrolysis of salt solutions is their high conductivity and increased resistance to acidic and caustic environments resulting from the electrolytic process. The known disadvantage of polymers, however, is their low selectivity for ionic species. That is, polymers transport protons as well as alkali ions, such as sodium ions, the result of which is an inefficient operation of the electrolytic cell.
The known advantage of ceramic materials is their excellent ion-conducting characteristics and good selectivity under certain conditions. Nasicon (sodium [Na] super ion conductor) materials, such as Na.sub.3 Zr.sub.2 PSi.sub.2 O.sub.12, have a known advantage over beta alumina, another sodium ion-conductors in that Nasicon is comparatively stable in water while beta alumina is not. Further, Nasicon materials are comparable to beta alumina in superiority as sodium ion-conductors. The disadvantage of Nasicon-type materials is that some Nasicon-type materials are unstable in acidic and caustic environments due to some species of the Nasicon being particularly subject to attack by acids and the like. Further, ceramic materials have a lower ion conductivity at lower temperatures as compared to polymeric materials. Thus, the combination of polymeric and ceramic materials may provide a more suitable material for use in electrolysis of salt solutions.
Given the specific disadvantages of polymeric materials, however, and the additional labor and cost of producing ceramic/polymeric composite materials, it would be advantageous to provide a ceramic material having superior ion-conducting and selectivity characteristics, while being comparatively stable in water and acid/caustic environments.